Honestly I've never heard of anyone thinking Marth was a female until I read through some of these posts.

Samus's gender I found out about after playing the first SSB I believe. It was like some rumor being passed around at my elementary school or something haha. Using Pikachu's Thunder attack on Samus revealed it to be true.

I actually knew both, but people kept telling me that Marth was a girl. I paused the game and looked at him, and I was still pretty sure he was a boy, but that did nothing to convince others otherwise.

I knew from the beginning. it's easy to tell. marths voice is deeper than a womans and samus's figure looks too much like a girls. i was fooled for a while on samus. but how could you think that marth is a girl?

Remember how you could see skeletons in SSB64 when people were electrocuted? Found out Samus was a female when my friends and I were zapping 'him' with electricity and then pausing it to see what he looked like without armor.

Both were revealed to me with SSBB. I was a Final Smash junkie, and Samus was(and is) one of my favorite video game characters of all time. Behind Ike and Cubone. One of those lasers, and I was grilled. Interpret that however you want. I didn't think about Marth at all until the mission where Marth guarded Zelda until she transformed into Sheik. I was in shock for hours.